The labels we attach to the past shape the way we think about history. Powerfully and problematically so, in the case of late medieval England. Historians are fond of pointing out that "the Hundred Years' war" and "the wars of the Roses" are misleading terms of convenience which give a spurious coherence to periods of enormous complexity. But it's still oddly disconcerting to find that this latest volume in the New Oxford History of England series starts in the middle of one and ends in the middle of the other.

Nor is Gerald Harriss's chosen century bounded by the familiar roll-call of royal deaths. In 1360 the warrior-king Edward III was a vigorous and commanding figure at the age of 48, still more than a decade away from the descent into senility which preceded his death in 1377. 1461, by contrast, did mark the end of a reign, when the holy fool Henry VI was deposed by his cousin Edward IV. But Henry lost his liberty rather than his life, and was subsequently (if briefly and unsuccessfully) restored to the throne by Warwick the Kingmaker.

The rejection of chronological convention is only the first of many ways in which this outstanding book prompts a closer look at what we think we know about the medieval world. Historians used to see the 14th and 15th centuries in England as a period of decline and decay before the glories of the 16th-century Renaissance. Debilitation was apparent politically, in the civil wars between York and Lancaster; culturally, in the corruption and stagnation of the medieval church; and socially, in the human suffering and economic dislocation caused by the Black Death.

That picture has now been comprehensively challenged, but the bewildering volume of new research means that it's no longer easy to know how to see the period as a whole. The late middle ages, it seems, were the best of times and the worst of times, an era so full of contradictions as to be almost incomprehensible. Aristocratic culture was infused with chivalric ideals, while knights fought barbarous wars for mercenary ends. The faithful poured thousands of pounds into the building of parish churches, yet anticlericalism found strident voice in parliament and a champion in John Wyclif. Structures of government could withstand the rule of a nine-month-old baby (the infant son on whom Henry V had not yet set eyes when he died in France in 1422) - but by the end of the century four kings, this baby among them, had lost the throne by violence in a period of only 30 years.

Harriss brings extraordinary scholarship to the task of illuminating a path through this demanding terrain. He has produced a survey of stunning breadth and crystalline judgment, ranging with equal authority over subjects as diverse as the technicalities of medieval ship-building and the liturgical implications of polyphony and plainsong. Some of the seeming contradictions in medieval society, it emerges, are more apparent than real - the tension between the ideals and practice of chivalry, for example, "which, as in Christianity, had always existed without invalidating its claims". Others, such as the potential conflict between descending and ascending theories of political authority - the relationship between the king's rights and those of his subjects - had a formative influence on English society across the period as a whole.

More than 500 years on, this issue - especially the relationship between war, taxation and the national interest - still has an immediate resonance. Kings could levy direct taxation only if it was deemed to be necessary for the good of the realm, circumstances usually understood to apply only in wartime. As Harriss wryly points out, this prescription was "not unduly inhibiting" for the crown, given that England and France were almost constantly at each other's throats. Nevertheless, the fact remains that the royal regimes of late medieval England, whatever their claims to rule by divine sanction, could not raise money for military campaigns abroad without first submitting the case for war to legal and political scrutiny in parliament. And if the campaigns failed - if the nation's money was spent, but the national interest was not served - leaders faced criticism of their integrity, competence and judgment, criticism which could and did bring governments down.

The question of continuity and change within the English state in fact gives the book its overarching framework. Harriss argues, with conviction and insight, that the after-effects of plague combined with sporadic political crisis and sustained military commitment to complete "the development from a feudally structured society to a politically integrated one". It was this period, rather than any revolution or evolution in government under the Tudors, which determined the functioning of the English constitution until the civil war.

Here Harriss will not go unchallenged: the task of pushing the Tudors out of the historical spotlight is a mammoth one. But he demonstrates with unanswerable clarity the bankruptcy of the traditional idea that the "middle ages" ended and "modern history" began with Henry VII's victory at Bosworth field. At more than 600 closely-argued pages, Shaping the Nation is not in any sense a light read. But as an insight into the complex reality of late medieval life, it is matchless.